The rotor of a direct drive generator for wind turbines has a diameter of more than 4 m, dependent on the generator power and torque. The generator rotor of the Siemens Wind Power direct drive turbines for example has an outer diameter of 4.2 m for the SWP 3.0-101 and an outer diameter of 6.5 m for the SWP 6.0-154 and a length of around 2.5 m.
The rotor consists of a front plate, a rotor ring with a certain yoke height and a bearing ring. The complete rotor is also called rotor housing because in the Siemens Wind Power direct drive turbines the rotor is located outside of the stator and acts therefore like a house. The rotor outer surface is directly in contact with the ambient air.
The rotor housing of all the direct drive Siemens Wind Power generators is a single welded and machined steel component. The rotor housing consists of two large cones, which are welded together with a forged steel ring and a rolled steel yoke. This rotor housing is then machined in a large CNC machine.
Previously, it was especially bearings in large DC and AC motors with few poles that risked current flow. In modern electric machines one should be aware of the risk of current flow in both the bearing and the machine's drive chain. This is due to the ever increasing use of frequency converters. In recent years there has been an increase in motors controlled by PWM inverters (variable frequency drives) in industrial HVAC, pumping, and processing equipment. VFD's induce voltage onto the shaft of the driven motor may cause pitting, fluting and finally bearing and motor failure.
Damaged bearings, for instance, can cause generator failures, which lead to unplanned downtime and costly repairs. A single month's wait for parts is unrealistically short considering the worldwide shortage of bearings and other key components. On top of lost revenue is the cost of repairing failed bearings due to for example new bearings, labour, slip rings, and other parts, but also enormous expense of renting and transporting the large crane needed for many repairs must be accounted for.
Bearing currents caused by stator-to-rotor capacitive coupling must be diverted from the shaft by providing a least resistance path to ground other than the bearing themselves.
Moreover, it is known to prevent damage of bearings due to induced shaft currents, by insulating the bearing from the shaft currents and/or by providing an alternative path for the shaft currents to flow. In current designs of direct drive generators, fiberglass laminates are used to isolate the bearing from shaft induced voltage. Fiber glass laminates acts as a good insulation for direct current and low frequency current to enter the bearing, but this is not useful for very high frequency currents, as the capacitance impedance reduces as frequency increases. In current designs shaft grounding brushes are also used to ground stray currents, but this is also not useful to ground very high frequency currents.
The problem addressed in this document is related to generator shaft currents where induced current causes damage to the main bearing of a wind turbine generator. Therefore an electrical insulation of the main bearing may be needed to avoid failure of the generator. Such an electrical insulation increases the effort for producing a direct drive generator. Hence, there may be a need to produce a direct drive generator with an outer rotor in an easier and cost reducing manner.